The present invention relates to composite electrodes and functional layers for electrode reactions for use with solid-state ionic devices, and solid oxide fuel cells in particular.
The following references are referred to herein by their numerical reference and the contents of each is incorporated herein by reference.
1. Erning, J. W., Hauber, T., Stimming, U. Wippermann, K., Catalysis of the electrochemical processes on solid oxide fuel cell cathodes, Journal of Power Sources 61 (1996) 205-211.
2. M. Watanabe, H. Uchida, M. Shibata, N. Mochizuki and K. Amikura, High performance catalyzedxe2x80x94reaction layer for medium temperature operating solid oxide fuel cells, J. Electrochem. Soc., vol. 141, (1994) 342-346.
3. Sahibzada, M., Benson, S. J., Rudkin, R. A., Kilner, J. A., Pd-promoted La0.6Sr0.4Co0.2Fe0.8O3 cathodes. Solid State Ionics 113-115 (1998) 285-290.
4. M. M. Murphy, J. Van herle, A. J. McEvoy, K. Ravindranathan Thampi, Electroless deposition of electrodes in solid oxide fuel cells, J. Electrochem. Soc., vol. 141 (1994) 30 L94-96.
5. Uchida et al. Shin-ichi Arisaka and Masahiro Watanabe, Paper B-IN-05 at 121st International Conference on Solid State Ionics (1999) 154-155.
Solid state ionic devices typically consist of a fully dense electrolyte sandwiched between thin electrode layers. It is known that the principal losses in most solid state ionic devices occur in the electrodes or the electrode/electrolyte interfaces. Therefore, minimization of these losses is critical to efficient operation of these devices.
Solid oxide fuel cells (SOFC) are theoretically very efficient energy conversion devices that have the potential of becoming a commercial product for numerous uses. A SOFC is a solid electrochemical cell which consists of a solid electrolyte impervious to gases, sandwiched between a porous cathode and a porous anode. Oxygen gas is transported through the cathode to its interface with the electrolyte where it is reduced to oxygen ions, which migrate through the electrolyte to the anode. At the anode, the ionic oxygen reacts with fuels such as hydrogen or methane and release electrons. The electrons travel back to the cathode through an external circuit to generate electric power.
The construction of conventional SOFC electrodes are well known. Electrodes are often applied as composites of an electron conducting material and an ion conducting material. For instance, an anode may consist of electronic conducting nickel (Ni) and ionic conducting yttria stabilized zirconia (YSZ) while the cathode may consist of a perovskite such as La1xe2x88x92xSrxMnO3xe2x88x92xcex4 (LSM) as the electron conducting material and YSZ as the ion conductor.
Conventional SOFCs exhibit high performance at operating temperatures of 1000xc2x0 C. However, such high temperature operation has disadvantages such as physical or chemical degradation of the construction materials. Therefore, it is desirable to reduce the operating temperature of a SOFC stack to a medium temperature of about 700xc2x0 C. However, at such medium temperatures, electrode reaction rates decrease significantly. Prior art efforts to increase electrode reactivity at lower temperatures have focussed on optimizing the electrode microstructure and by introducing catalytic materials into the electrode structure.
It is well known to provide an activated surface on the fuel cell electrodes by means of a catalyst to aid the electrochemical process. Nickel is commonly used as a catalyst on the anode side for oxidation of fuel. On the cathode side, ceramic cathode materials typically used in SOFCs, such as perovskites have a high activation energy for oxygen reduction. Therefore, the activation energy may be reduced for the oxygen reduction reaction by adding noble metals such as Au, Ag, Pt, Pd, Ir, Ru and other metals or alloys of the Pt group. Erning et al. [1] reported that addition of highly dispersed noble metals ( less than =0.1 mg/cm2) lowers the activation energy of the oxygen reduction reaction at the cathode of an SOFC. M. Watanabe [2] also found that the anodic polarization resistance and its activation energy were greatly decreased by loading only a small amount of catalyst such as Ru, Rh, and Pt onto a samaria-doped ceria (SDC) anode. A large depolarizing effect was also observed with a Pt-catalyzed LSM cathode, especially at high current densities. Sahibzada et al. [3] has recently reported that LSCF electrodes which were impregnated with small amounts of Pd resulted in 3-4 times lower cathodic impedance in the temperature range 400 to 750xc2x0 C. The overall cell resistance decreased 15% at 6500xc2x0 C. and 40% at 550xc2x0 C.
For economic reasons, noble metal catalysts are applied in very small amounts to catalyze the electrochemical process at electrodes. The catalysts are conventionally impregnated in the pores of the electrode by a filtration or a chemical process. The impregnation process is frequently followed by a binding process where a binder is superimposed on the deposited particles to provide a secure and durable attachment of the coating with the base material. U.S. Pat. Nos. 3,097,115; 3,097,974; 3,171,757 and 3,309,231 disclose such conventional impregnating processes for porous electrodes.
The catalysts may also be applied by common electroless deposition techniques for Ni, Pd and Ag [4] and replacement plating, as disclosed in U.S. Pat. No. 3,787,244. In this process, an acidic plating solution containing a salt of a noble metal catalyst is forced through the pores of a nickel electrode substrate and the noble metal ions from the dissolved salt replace a thin layer of the nickel surface within the pores.
It is known [1] to form highly dispersed catalyst layers with an amount of less than 0.1 mg/cm2 from aqueous solutions of Pt, Pd, Ir or Ru salts. A few drops of these solutions were applied onto the electrolyte surface. After drying, the salts were either reduced to metal form by heating under hydrogen (Pt and Pd) or oxidized by heating under air (Ir and Ru), Most recently, Uchida et al. [5] applied nanometer-sized noble metal catalysts to both anode and cathode resulting in appreciably lower overpotential ohmic resistance.
Singheiser (EP 424813) discloses an intermetallic compound layer (0.5-5 xcexcm) contains 2-70 wt. % of a noble metal such as Pt, Ag or Pd which can be used between electrolyte and electrodes, or to connect electrically two fuel-cells. It is claimed that the fuel cell can be operated at a lower temperature due to higher electrode conductivity.
Because of the cost of noble metals, the application of noble metals in SOFC electrodes so far are mainly limited to its catalytic abilities. All recent efforts have been to add very fine particles of the catalyst in order to maximize the three phase boundary of the catalyst, the gas phase and the electrolyte. The catalyst is either applied as a very thin layer at the electrolyte/electrode boundary or is widely dispersed throughout the electrode.
In U.S. Pat. No. 5,543,239 issued to Virkar et al., an electrocatalyst is incorporated into a electrode microstructure that is claimed to improve the performance of a solid state ionic device by providing a catalyst and by improving electrical conductance. In this disclosure, a porous ionic conductor is applied to a dense electrolyte substrate. An electrocatalyst is then introduced into the porous matrix to produce electrical continuity and a large three phase boundary line length. As a result, the electrocatalyst is applied as a thin layer of small particles over the ionic conductor.
The electrode disclosed by Virkar et al., however, does not solve the problem of electrode instability. It is known that vapor loss of noble metals occurs at even medium SOFC operating temperatures. According to the Thomson-Freundlich (Kelvin) equation, an important aspect of the vapor pressure difference across a curved surface is the increase in vapor pressure at a point of high surface curvature. Thus, the smaller the particle size, the higher the vapor pressure. This could cause significant vapor loss for small noble metal particles at SOFC operating temperatures.
Furthermore, higher vapor pressure at the particle surface and lower vapor pressure at a neck between two particles makes smaller particles much easier to be sintered. Thus, the microstructure of an electrode with submicronic noble metal ( less than 0.5 xcexcm) particles is not stable at medium to high SOFC operating temperatures, and especially when the electrode handles high current.
Furthermore, a thin electronic conducting layer at the electrode will have large ohmic resistance at the electrode which limits the current carrying capacity of the electrode. As shown in the current-voltage curves of the Virkar et al. patent, the experimental current is limited to 0.5 A/cm2 for the Pt/YSZ and LSM/YSZ cathodes disclosed therein.
Therefore, there is a need in the art for a composite electrode which mitigates the limitations of the prior art, allowing higher performance solid state ionic devices and solid oxide fuel cells in particular.
The present invention is directed at an electrode having an improved microstructure which achieves a high density of active electrochemical reaction sites between the electrolyte and electrode and incorporates electrocatalytic materials such as noble metals into the electrode in an intimate fashion. As well, the improved microstructure also improves long-term structural stability of the cell by reducing the effects of noble metal catalyst sintering and vapor loss. The electrode may be incorporated into any solid state electrochemical devices such as oxygen pumps, membranes and sensors, solid state batteries or solid oxide fuel cells. The electrode of the present invention may be either a cathode or an anode.
Accordingly, in one aspect of the invention, the invention comprises an electrode forming part of a solid state electrochemical device, said electrode bonded to a dense electrolyte layer and comprising a porous three-dimensional solid phase structure comprising:
(a) an electrocatalytic phase comprising a plurality of electrocatalytic particles;
(b) an ionic conducting phase comprising a plurality of ionic conductor particles;
wherein said electrical conducting phase and ionic conducting phase are interspersed and wherein the mean size of said noble metal particles is substantially equal to or larger than the mean size of said ionic conducting particles.
The electrode of the present invention is formed by mixing ceramic ion conductor particles and noble metal electrocatalyst particles into a composite electrode which is then applied to a dense electrolyte substrate by screen printing or by similar well-known methods. The resulting electrode microstructure is highly porous and includes very long three-phase boundaries, direct ion conducting channels from the catalytic sites to the electrolyte and direct electron conducting channels through the electrode to the catalytic sites. The electrocatalyst particles are preferably comprised of a noble metal and are preferably larger than the ion conductor particles which results in a morphology where the ion conductor particles pin the boundaries of the noble metal particles. The relatively large noble metal particle size reduces vapor loss at elevated temperatures while grain boundary pinning reduces or prevents sintering or coalescing of the noble metal particles.
In one embodiment, the ion conductor particles may comprise ceramic particles which may preferably be yttrium stabilized zirconia and the noble metal particles may comprise palladium. Those skilled in the art will be aware of other materials which will function as ion conducting particles and as electrocatalytic particles.
In one embodiment, the invention may comprise an electrode comprising (a) an electrode functional layer for use in a solid state electrochemical device, said layer comprising a porous three dimensional structure comprising linked particles of an electrocatalytic material and linked particles of an ionic conductor wherein the median size of the electrocatalyst particles is approximately equal to or larger than the median size of the ion conducting particles; and (b) a long range conducting electrode layer that is applied on top of the functional layer. In a planar SOFC, long range conductivity refers to horizontal conductivity between the ribs of the interconnect plate, rather than the short range vertical conducting path through the ceramics. The conducting layer may comprise electronically conductive metal oxides such as lanthanum cobaltate.
In another aspect, the invention comprises a solid state electrochemical device comprising a porous anode, a dense electrolyte and a cathode comprising a porous three-dimensional structure comprising linked particles of an electrocatalytic material and linked particles of an ionic conductor wherein the mean or median size of the electrocatalyst particles is larger than the mean or median size of the ion conducting particles. The solid state electrochemical device may be a solid oxide fuel cell.
In another aspect of the invention, the invention is a method of forming an electrode for use in a solid state electrochemical device having a dense electrolyte layer comprising the steps of:
(a) mixing electrocatalytic particles with ion conducting particles where mean or median size of the electrocatalytic particles is substantially equal to or larger than the mean or median size of the ion conducting particles; and
(b) creating a porous three-dimensional structure bonded to the dense electrolyte layer, said structure comprising linked particles of the noble metal particles and linked particles of the ionic conductor.
In one embodiment, a further conducting layer of metal oxides may be applied but are not presintered to the electrode. The metal oxides may comprise lanthanum cobalt oxide.